Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuels, as well as portable power storage, such as lithium-ion batteries.
In general, fuel cell technology includes a variety of different fuel cells, such as alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and enzyme fuel cells. Fuel cells generally run on hydrogen (H2) fuel, and they can also consume non pure hydrogen fuel. Non pure hydrogen fuel cells include direct oxidation fuel cells, such as direct methanol fuel cells (DMFC), which use methanol, or solid oxide fuel cells (SOFC), which use hydrocarbon at high temperature. Hydrogen fuel can be stored in compressed form or within compounds such as alcohols or hydrocarbons or other hydrogen containing materials that can be reformed or converted into hydrogen fuel and byproducts. Hydrogen can also be stored in chemical hydrides, such as sodium borohydride (NaBH4), that react with water or an alcohol to produce hydrogen and byproducts. Hydrogen can also be adsorbed or absorbed in metal hydrides, such as lanthanum pentanickel (LaNi5) at a first pressure and temperature and released to fuel a fuel cell at a second pressure and temperature.
Most hydrogen fuel cells have a proton exchange membrane or polymer electrolyte membrane (PEM), which allows the hydrogen's protons to pass through but forces the electrons to pass through an external circuit, which advantageously can be a cell phone, a personal digital assistant (PDA), a computer, a power tool or any device that uses electron flow or electrical current. The fuel cell reaction can be represented as follows:
Half-reaction at the anode of the fuel cell:H2→2H++2e−Half-reaction at the cathode of the fuel cell:2(2H++2e−)+O2→2H2O
Generally, the PEM is made from a polymer, such as Nafion® available from DuPont, which is a perfluorinated sulfonic acid polymer having a thickness in the range of about 0.05 mm to about 0.50 mm, or other suitable membranes. The anode is typically made from a Teflonized carbon paper support with a thin layer of catalyst, such as platinum-ruthenium, deposited thereon. The cathode is typically a gas diffusion electrode in which platinum particles are bonded to one side of the membrane.
For DMFC, the chemical-electrical reaction at each electrode and the overall reaction for a direct methanol fuel cell are described as follows:
Half-reaction at the anode:CH3OH+H2O→CO2+6H++6e−
Half-reaction at the cathode:1.5O2+6H++6e−→3H2O
The overall fuel cell reaction:CH3OH+1.5O2→CO2+2H2ODMFCs are discussed in U.S. Pat. Nos. 4,390,603 and 4,828,941, which are incorporated by reference herein in their entireties.
In a chemical metal hydride fuel cell, sodium borohydride is reformed and reacts as follows:NaBH4+2H2O→(heat and/or catalyst)→4(H2)+(NaBO2)Suitable catalysts for this reaction include platinum and ruthenium, and other metals. The hydrogen fuel produced from reforming sodium borohydride is reacted in the fuel cell with an oxidant, such as O2, to create electricity (or a flow of electrons) and water by-product, illustrated above. Sodium borate (NaBO2) by-product is also produced by the reforming process. A sodium borohydride fuel cell is discussed in U.S. Pat. No. 4,261,956, which is incorporated by reference herein in its entirety.
As noted above, the products/byproducts of the fuel cell reactions include gasses such as hydrogen and carbon dioxide which may increase the internal pressure within the devices in which they are stored or generated. Additionally, as fuel is consumed in the fuel cell system a vacuum may be generated impairing the further flow of fuel from fuel supplies and/or cartridges to the fuel cell. Thus, various relief valves are known in the art for relieving these issues. However, these valves often involve numerous parts (springs, o-rings, elastomers, etc.) and a concern still remains regarding the reliability as well as the economics of these valves given that components such as fuel cartridges and supplies may be disposable. To a certain extent, this need for improved relief valves for fuel cell systems has been addressed by commonly owned, co-pending U.S. published application nos. 2007/0114485 [BIC-017.D1] as well as parent U.S. patent application Ser. No. 12/674,205 [BIC-112], which are incorporated herein by reference in their entireties. Nonetheless, there still exists the need for reliable relief valves and valves that can vent pressurized systems and can allow relief from a vacuum.